Data are commonly transmitted among the stations of a network in "packets." Each packet is a group of bits transmitted in a short contiguous burst. Between packets of a first conversation, packets of other conversations can use the channel. For instance, a voice transmission is encoded as about 10,000 bits per second. Each second's bits might be broken into 20 packets of 500 bits each, and each packet transmitted over the channel at 2 million bits per second. Thus, a single transmission channel could transmit about 100 conversations simultaneously. The packets of each conversation are interleaved with packets of other conversations.
In wireless transmission of digital data, for instance between cellular phones, a receiver must distinguish noise from signals carrying data intended to be received. The noise can come from any number of sources, including other distant data transmitters operating on the same frequency, and non-data transmitters such as microwave ovens, heart monitors, or video devices.
Since several transmitters may be sharing a single transmission frequency, each transmitter must "listen" for a moment before beginning to transmit, to ensure that no other too-close transmitter is currently transmitting. If the transmitter detects another packet in progress, and determines that the other transmitter is geographically close (so that starting another packet would interfere with the packet already in progress), the transmitter defers a moment, and then "listens" again to determine whether the channel is clear for transmitting. The transmitter faces the same challenge as a receiver--the transmitter distinguishes between noise and signal, and between weak signals from distant transmitters (in which case transmission may proceed) and stronger signals from nearer transmitters (in which case the packet must be deferred). This distinguishing between data signal and noise to determine when to transmit is called "carrier sense media access," or CSMA.
Proper carrier sensing is important to high-throughput and efficiency in data transmission over wireless networks. If the transmitter is "too polite," that is, if it is too conservative in determining whether another packet is in progress, too many packets will be deferred. A "too polite" transmitter will spend more time than necessary waiting. If the transmitter is "too vocal," then the other packet already in progress and the vocal transmitter's message will collide, and one or both will be spoiled by the resultant interference. The polite/vocal characteristic is expressed as a value called the "carrier sense defer threshold:" if the energy of the transmission's data carrier exceeds the carrier sense defer threshold, the transmitter defers, else the transmitter transmits. When the carrier sense defer threshold is too low, the transmitter is too vocal; when the defer threshold is too high, the transmitter is too polite.
FIG. 1 is a simplified representation of transmission in a carrier sensing wireless network. Several packets may be in transmission simultaneously, so long as they are geographically separated. The symbols used in FIGS. 1 and 2 are as follows:
T represents a transmitting station PA1 R represents a receiving station PA1 Q represents a quiescent station PA1 --.fwdarw.(solid arrow) represents a transmission in progress PA1 --- .fwdarw.(dashed arrow) represents a transmission deferred PA1 --X (x-headed arrow) represents an interfering transmission, causing a packet collision PA1 .largecircle. (large circle) shows the horizon of transmission power and carrier sense around a transmitter
Transmitter T.sub.1 is transmitting a packet to receiver R.sub.1, T.sub.2 to R.sub.2, T.sub.3 to R.sub.3, and T.sub.5 to R.sub.5 (these transmissions in progress are shown by solid arrows 121, 122, 123, and 125). Transmitter T.sub.4 would like to transmit to receiver R.sub.4. Circles 102-110 around each of the transmitters describe the area in which the signal can be received, and also the area in which other transmitters defer until the previous packet is complete. Thus, because T.sub.4 is inside T.sub.3 's circle 106, the T.sub.4 -R.sub.4 packet is deferred (noted by the dotted-line arrow 124), until the T.sub.3 -R.sub.3 packet 123 is complete. Note that T.sub.4 defers even though R.sub.4 lies outside T.sub.3 's circle 106 (T.sub.3 would not interfere with R.sub.4 's receipt of T.sub.4 's message); nonetheless, T.sub.4 defers because it can "hear" T.sub.3 's transmission, and therefore assumes that T.sub.4 's transmission would interfere with R.sub.3 's receipt of T.sub.3 's message. T.sub.5 is transmitting (arrow 125) to R.sub.5, but this packet collides (x-headed arrow 132) with the T.sub.2 -R.sub.2 packet: because T.sub.2 and T.sub.5 are out of range of each other, neither observes the other's packet, and neither defers. This "hidden transmitter problem" occurs where two transmitters are out of range of each other, but the intended receiver is within range of both.
Recently, the problem of carrier sensing has been further complicated by two developments. First, an increasing number of wireless networks involve mobile receivers or mobile transmitters. Mobility results in variations in the strength of the received signal, which in turn blurs the difference between a weak data signal (because transmitter and receiver have moved far apart) and a strong noise signal (for instance, from another transmitter-receiver pair somewhat farther away). Second, the FCC has recently allowed transmission in segments of the spectrum already occupied by microwave ovens and similar devices. In these applications, a receiver (and a transmitter using carrier sensing to decide whether to transmit) extract a signal from out of undesired random environmental noise, where the noise signal may be stronger than the data signal.